1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device using a variable resistance film, and to a method of manufacturing the same.
2. Description of the Related Art
A flash memory has been known as an electrically rewritable nonvolatile memory. Recently, various cross-point cells are developed in order to achieve high performance and scale reduction of a memory cell. A memory cell having the following configuration is developed as one of the cells (e.g., see S. B. Herner et al., “Vertical p-i-n polysilicon diode with Antifuse for Stackable Field-Programmable ROM”, IEEE Electron Device Letters, Vol. 25, No. 5, May 2004). According to the configuration of the memory cell, a diode functioning as a rectifier and a fuse formed of an insulating film are series-connected to a cross-point of a bit line and a word line. However, the memory cell is a destruction type cell; for this reason, data is not rewritable.
Various resistive random access memories (ReRAM) are developed as a rewritable cross-point cell. For example, one is an ReRAM configuring a memory cell, which comprises a serial circuit composed of a variable resistance element, a diode and a switching element (e.g., see Jpn. Pat. Appln. KOKAI Publication No. 2006-351779). The other is an ReRAM configuring a memory cell, which comprises a serial circuit composed of a variable resistance element and a diode (e.g., see Jpn. Pat. Appln. KOKAI Publication No. 2006-295177). The ReRAM configuring a memory cell, which comprises a serial circuit composed of a variable resistance element and a diode, has attracted much interest due to the following reason. Namely, a memory cell is arranged at a cross point of a word line and a bit line, and thereby, a cross-point cell array is easily configured.
In order to form a cross-point cell array, a material configuring a diode and a resistive change material configuring a variable resistance element are successively stacked. Thereafter, the materials are patterned using reactive ion etching (RIE) to form a memory cell. In order to secure reverse-bias leak resistance, a diode having a thick film is required taking an elongation of a depletion layer into consideration. For this reason, the memory cell pattern after etching has a high aspect with scale reduction of the device. As a result, there is a problem that the pattern is destroyed.
Further, the following problem arises when etching is carried out using RIE. Specifically, a leak occurs in the side of a top electrode, a diode and a bottom electrode resulting from the interface state of the sidewall of the diode. Furthermore, the following problem arises because the variable resistance element is etched using RIE. Specifically, on the sidewall, a device characteristic is degraded resulting from destruction of the composition and damage by RIE such as dangling bonds. Therefore, it is desired to provide a nonvolatile semiconductor memory device which can prevent characteristic deterioration of a diode and a variable resistance element.